The Cassie and Wenzel phenomena, occur classically when three phases are in contact with one another. For example, one can have one solid and two liquid phases in contact, with the two liquid phases being different in their hydrophobicity. In the body, the respective states lead to the formation and retention on an implant of a liquid hydrophilic film in the Cassie state and retention of tissue (containing lipids) in the Wenzel state. These are clinically useful attributes for localizing an implant within living tissue.
Shear is motion of an implant parallel to a tissue surface, and peal is motion of an implant perpendicular to a tissue surface. Clinically, an implant with high shear force resists migration in the body and an implant with low peal force can be repositioned easily by the clinician during the surgical procedure.
The interaction of a solid textured surface with water in a gaseous environment is described by the Cassie-Baxter model. In this model, air is trapped in the microgrooves of a textured surface and water droplets rest on a compound surface comprising air and the tops of micro-protrusions. The importance of a fractal dimension between multiple scales of texture is well recognized and many approaches have been based on the fractal contribution, i.e., the dimensional relationship between different scales of texture. However, regardless of the material (organic or inorganic) used and geometric structure of the surface texture (particles, rod arrays, or pores), multiple scales of texture in combination with low surface energy has been required to obtain the so called super hydrophobic surfaces.
Super hydrophobicity is variously reported as a material exhibiting a contact angle with water that is greater than the contact angles achievable with smooth but strongly hydrophobic materials. The consensus for the minimum contact angle for a super hydrophobic substance is 150 degrees.
A hydrophobic surface repels water. The hydrophobicity of a surface can be measured, for example, by determining the contact angle of a drop of water on a surface. The contact angle can be measured in a static state or in a dynamic state. A dynamic contact angle measurement can include determining an advancing contact angle or a receding contact angle with respect to an adherent species such as a water drop. A hydrophobic surface having a large difference between advancing and receding contact angles (i.e., high contact angle hysteresis) presents clinically desirable properties. Water or wet tissue can travel over a surface having low contact angle hysteresis more readily than across a surface having a high contact angle hysteresis, thus the magnitude of the contact angle hysteresis can be equated with the amount of energy needed to move a substance across a surface in shear. In clinical applications, a high contact angle reduces the mobility of the implant in situ.
The classic motivation from nature for surface texture research is the lotus leaf, which is super hydrophobic due to a hierarchical structure of convex cell papillae and randomly oriented hydrophobic wax tubules, which have high contact angles and low contact angle hysteresis with water and show strong self-cleaning properties. A lesser known motivation from nature is the red rose petal, with a hierarchical structure of convex cell papillae ornamented with circumferentially arranged and axially directed ridges, which have a moderate contact angle and high contact angle hysteresis.
The contact angle is a measure of the amount of water directly in contact with the implant surface, while the contact angle hysteresis is an inverse measure of the degree to which water is mobile on a surface. The natural evolutionary motivation for each of these states is quite distinct.
In the case of the lotus leaf, and botanical leaves generally, minimal contact with water and high water mobility results in preferential adherence of the water to particulate contaminants, which are cleared from the leave as the water runs off. This serves to reduce light absorbance by surface contaminants, and increase photosynthetic efficiency.
In the case of the rose petal, and botanical petals generally as opposed to leaves, most pollinators are attracted to high tension water sources which provide ready accessibility without drowning the insect.
Thus, high contact angle paired with high contact angle hysteresis is preferred where the evolutionary stimulus is reproduction in botanicals, and high contact angle paired with low contact angle hysteresis is preferred where the evolutionary stimulus is metabolism and growth.
Considering for a moment a single texture scale, when water is placed on a textured surface it can either sit on the peaks of the texture or wick into the valleys. The former is called the Cassie state, and the later the Wenzel state. When the Wenzel state is dominant, both the contact angle and contact angle hysteresis increase as the surface roughness increases. When a roughness factor exceeds a critical level, however, the contact angle continues to increase while the hysteresis starts decreasing. At this point, the dominant wetting behavior changes, due to an increase in the amount of air trapped at the interface between the surface and water droplet. In the present context, the gaseous state is replaced with a hydrophobic state; for example, a lipid. The hydrophobic state may be a liquid or a solid derived from the host tissue.
In mixed Wenzel-Cassie states it is possible to have high contact angle and high contact angle hysteresis. However texture alone is only one aspect, the hydrophobicity of a textured solid relative to the interacting environment is also important.
Water possesses a dipole structure which makes it attractive to any other substance that is charged. Implantable molecules with a charge surplus localized at a specific location on the molecule renders that molecule hydrophilic. In the case of polymers, the charges can associate, and the bulk substance can possess a macroscopic surface charge. And in such macroscopic assemblages, these materials are strongly water attractive. And when those macroscopic charge localities are associated with surface texture, then the implant material becomes super hydrophilic.
Thus, while it is generally advantageous for an implant to be hydrophilic, and associate readily with water in living tissue, this association creates a fluid surface between the implant and the tissue, which acts as a lubricant. Generally, it is disadvantageous for an implant to move from a position determined by a clinician, and generally it is disadvantageous for an implant to require suture or other physical means of localization. Therefore, utilization of an in situ analog to the Cassie-Wenzel state to localize an implant in living tissue is clinically desirable.